The unusual properties of the prefrontal cortex (PFC) dopamine (DA) system contribute to the unusual sensitivity of PFC and the cognitive functions that depend on it to environmental and genetic variations. (Those cognitive functions are called 'executive functions' [EFs] and are important for reasoning, planning, problem-solving, and self-control.) We propose to study the effects of an environmental factor (mild stress) on EFs, testing our predictions of how and why those effects differ by biological factors (hormones and genotype). To test our hypotheses concerning the underlying mechanism, we will try to model the effects of mild stress on EFs pharmacologically. Two ways in which the DA system in PFC is unusual are: (1) It relies heavily on the catechol-O-methyltransferase (COMT) enzyme for clearing released DA (other brain regions rely heavily on the dopamine transporter). (2) Mild stress markedly increases DA in PFC (but not in other brain regions). Animal studies find a sex difference in that mild stress aids performance on cognitive tasks that require PFC in males, but hinders it in females. We propose (a) a new interpretation of that, (b) to see if this sex differene is also true in humans, and (c) to put our new interpretation to the test. We hypothesize that mild stress exerts its effect on cognition, and produces the sex difference in that, by increasing DA in PFC (rather than the mechanism being solely by increasing cortisol and affecting hippocampal functioning). If we are correct, then if we model this by pharmacologically increasing DA in PFC, we should be able to replicate the sex difference. There are two common variants of the gene that codes for the COMT enzyme, one codes for a much slower COMT enzyme than the other, clearing DA from PFC much more slowly. With a slower enzyme increasing the level of DA in PFC, the increase in PFC DA due to stress could well result in too much DA in PFC, impairing EFs. Thus we predict opposite effects of stress on cognition by COMT genotype, and we predict that by pharmacologically increasing DA in PFC, we should be able to replicate that (those homozygous for the gene coding for the faster enzyme should benefit, but those homozygous for the other version should show impaired EFs). We thus hope to put our causal model to the test by attempting to reproduce the opposing effects of mild stress in males and females and by genotype without stress but just by increasing DA in PFC. If our hypotheses are borne out, gender and genotype should be taken into account when considering the best environment and best drug dosage for aiding a person's self-control and creative problem-solving, and for minimizing a person's impulsive tendencies when trying to prevent or treat mental health disorders (such as addictions) in which EFs and the DA system in PFC are implicated and when teaching or designing a work environment.